Different lawyer has different price, but I think it may just less than the two times your filing fee.
As far as the document which you need to sent is considered, I think you need to file: except what you list, all passport information, SS#. Besides the I-131, copy of I-485,all others may not show on your cover letter, but you must attach them.
As far as the document which you need to sent is considered, I think you need to file: except what you list, all passport information, SS#. Besides the I-131, copy of I-485,all others may not show on your cover letter, but you must attach them.